A voltage controlled oscillator is an oscillator that changes its frequency according to a control voltage applied to its control input node. Voltage controlled oscillators (VCO's) have many applications in generating an oscillating voltage whose frequency is dependent upon a control voltage. One typical application of a VCO is in a phase locked loop (PLL) in which the output of the VCO is locked to an input frequency and the control voltage of the VCO is used as a reference voltage for other circuits.
In some designs, the total VCO frequency range is divided into a number of sub-ranges, allowing the VCO to operate with a lower control voltage gain. The lower VCO gain makes the VCO less susceptible to unwanted noise from the phase detector and PLL loop filter.
In some cases, the VCO may operate near the boundary of the selected frequency sub-range, causing the VCO gain to vary from the nominal value. This effect could lead to phase lock loop (PLL) instability or phase noise degradation. In addition, if the VCO nominally operates near the boundary of a frequency sub-range, operating temperature or voltage changes may make it necessary for the VCO to change sub-ranges during operation. This is undesirable as it would cause a large disturbance on the output signal of the VCO, thereby, affecting the system that uses the VCO.
FIG. 1 shows a prior art implementation of a VCO in a phase locked loop system. A buffer frequency divider block samples the output of the oscillator and feeds it back to the digital control block for comparison. The digital control block determines the center frequency of the VCO by selecting which capacitors are switched on for each half of the oscillating circuit. This implementation allows for frequency adjustments in discrete steps. The selection process may also include varying the control voltage to ensure that the VCO can oscillate at the desired target frequency when the switches are in a particular state. A voltage control node on the VCO is used to control the two varactor voltages. By changing the varactor voltages, the value of their capacitance changes to finely adjust the VCO to the desired operating frequency.